Epoxides are important intermediates for chemicals synthesis. They are precursors to large volume commodity chemicals such as ethylene glycol and propylene glycol that can be used as solvents, polymers such as polypropylene oxide, and many enantiomeric molecules that are intermediates for pharmaceutical and natural products synthesis. They are produced by selective oxidation of the corresponding alkenes via insertion of an oxygen atom across the carbon-carbon double bond. The common oxygen sources are peroxides, hydroperoxides, oxychlorides, and oxometal complexes. While effective, these reagents are expensive and their use is not environmentally friendly, generating significant amounts of unwanted byproducts. A much more desirable oxidant is molecular oxygen. To date, however, success in epoxidation with molecular oxygen is limited to activated terminal alkenes or alkenes without allylic hydrogens, such as ethylene and butadiene. (See R. B. Grant, and R. M. Lambert, Mechanism of the silver-catalyzed heterogeneous epoxidation of ethylene, Journal of the Chemical Society, Chemical Communications (1983) 662-3 and J. W. Medlin, J. R. Monnier, and M. A. Barteau, Deuterium Kinetic Isotope Effects in Butadiene Epoxidation over Unpromoted and Cs-Promoted Silver Catalysts, Journal of Catalysis 204 (2001) 71-76). Epoxidation of, for example, propene, could be achieved with reasonable yields only by using oxidants such as nitrous oxide or hydrogen peroxide. (See E. Ananieva and A. Reitzmann, Direct gas-phase epoxidation of propene with nitrous oxide over modified silica supported FeOx catalysts, Chemical Engineering Science 59 (2004) 5509-5517; T. Thoemmes, S. Zuercher, A. Wix, A. Reitzmann, and B. Kraushaar-Czarnetzki, Catalytic vapour phase epoxidation of propene with nitrous oxide as an oxidant, Applied Catalysis, A: General 318 (2007) 160-169; and L. Y. Chen, G. K. Chuah, and S. Jaenicke, Propylene epoxidation with hydrogen peroxide catalyzed by molecular sieves containing framework titanium, Journal of Molecular Catalysis A: Chemical 132 (1998) 281-292.)
There has been limited success in the epoxidation of unactivated alkenes with molecular oxygen because it is such a demanding reaction. Known commercial processes are the Ag catalyzed epoxidation of ethylene and butadiene. (See R. B. Grant and R. M Lambert, J. Chem. Soc., Chem. Commun., 1983, 662 and J. Will Medlin, John R. Monnier and Mark A. Barteau, J. Catal., (2001) 204, 71). These catalytic processes however fail when the alkene possesses allylic hydrogen. Recently, there has been extensive exploration using supported Au catalyst for alkene epoxidation with molecular O2. These studies fall into three classes, those with molecular O2 alone, those with the addition of peroxy initiator, and those that require a sacrificial reductant. Very low yields of propene epoxidation were observed using H2O, O2, and C3H6 over Au/TiO2 catalysts. Turner et. al. reported epoxidation of styrene to benzaldehyde, styrene epoxide and acetopheneone over Au55 clusters. (See M. Ojeda and E. Iglesia, Chem. Commun. (Cambridge, U. K.), 2009, 352 and M. Turner, V. B. Golovko, O. P. H. Vaughan, P. Abdulkin, A. Berenguer-Murcia, M. S. Tikhov, B. F. G. Johnson and R. M. Lambert, Nature (London, U. K.), 2008, 454, 981). However, the activity and the selectivity for epoxide were low. The addition of a peroxy initiator accelerated the epoxidation reaction, but the product distribution appeared to be very solvent dependent. (See M. D. Hughes, Y.-J. Xu, P. Jenkins, P. McMorn, P. Landon, D. I. Enache, A. F. Carley, G. A. Attard, G. J. Hutchings, F. King, E. H. Stitt, P. Johnston, K. Griffin and C. J. Kiely, Nature (London, U. K.), 2005, 437, 1132). Perhaps the most intensely studied system is one in which H2 was included as a sacrificial reductant, and the catalysts used were Au/TiO2, Au/TS-1 and Au/MCM-41 and Au—Ba/Ti-TUD. (See E. E. Stangland, K. B. Stavens, R. P. Andres and W. N. Delgass, J. Catal., 2000, 191, 332; A. K. Sinha, S. Seelan, S. Tsubota, and M. Haruta, Topics in Catalysis, 2004, 29, 95; and J. J. Bravo-Suarez, K. K. Bando, J. Lu, M. Haruta, T. Fujitani and S. T. Oyama, J. Phys. Chem. C, 2008, 112, 1115). The proposed mechanism for these catalysts involves the formation of H2O2 on the Au active site and the migration of the peroxide onto neighboring Ti to form Ti hydroperoxy species which can donate an [O] atom to propylene to form propylene epoxide. (See J. J. Bravo-Suarez, K. K. Bando, J. Lu, M. Haruta, T. Fujitani and S. T. Oyama, J. Phys. Chem. C, 2008, 112, 1115).
A recent report by Ketchie et. al. suggested an alternate route to generate H2O2 without using H2 (See Ketchie, W. C., Murayama, M., and Davis, R. J., Topics in Catalysis, 44 (1-2), 307 (2007). They found that during the Au-catalyzed CO oxidation in water at ambient temperature, H2O2 was formed in the aqueous phase.